Venturi cluster, and burners and methods employing such cluster

ABSTRACT

A burner arrangement includes a venturi cluster including a plurality of venturis arranged for parallel flow. The multi venturi arrangement utilizing pressurized fuel as the inducing fluid to induce a flow of air enables the provision of an ultra fuel lean premix of fuel and air. A central burner tube which extends outwardly beyond the delivery end of a primary burner tip and mounts a relatively small capacity nozzle at a substantial distance from the delivery end of the burner tip enabling the ultra fuel lean mixture to expand and slow down such that its linear speed does not exceed the flame speed of the mixture prior to by the flame of the spaced nozzle. A deflector may be positioned adjacent the nozzle to assist in stabilizing the flame after the expansion and slowing process has been completed.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a divisional of and claims priority pursuant to 35U.S.C. § 120 from co-pending application Ser. No. 10/759,323, filed Jan.16, 2004, which application Ser. No. 10/759,323 in turn is a divisionalof and claims priority pursuant to 35 U.S.C. § 120 from co-pendingapplication Ser. No. 09/874,383, filed Jun. 4, 2001, which applicationSer. No. 09/874,383 in turn claims priority pursuant to 35 U.S.C. §119(e) from provisional application Ser. No. 60/221,087, filed Jul. 27,2000. The entireties of the disclosures of said prior applications Ser.Nos. 10/759,323, 09/874,383 and 60/221,087 are hereby specificallyincorporated herein by this specific reference thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to venturis which induce the flow of afluid when an inducing flow of another fluid is passed therethrough. Theinvention further relates to industrial burners, and in particular toburners which utilize venturis to induce the flow of one or more of thecomponents of a combustible mixture and thereby create such mixture forintroduction into a combustion zone. The invention also relates toburner devices capable of creating and handling oxygen rich combustiblemixtures.

2. The State of the Prior Art

Venturi devices for inducing the flow of one fluid (the induced fluid)by flow of another fluid (the inducing fluid) are known. These devicesgenerally consist of a tube which has an inlet end, a throat area and anoutlet end. Generally speaking, the throat has a smaller flow area thanthe inlet end whereby to provide a low pressure area at the throat. Theinducing fluid flows through the tube from the inlet end of the venturito the outlet end, and a source of the induced fluid is in fluidcommunication with the low pressure area created in the throat of thedevice by the flow of the inducing fluid. Thus, the induced fluid isdrawn into the throat and mixes with the inducing fluid.

Venturi devices are particularly useful in burners where a flow of fluidfuel is used to induce a flow of air to thereby create a mixture of thefuel and the air in the venturi. Sometimes, however, it is useful to usethe combustion air to induce a flow of the fuel. Alternatively, a flowof air or fuel through the venturi may be used to induce a flow ofrecirculated flue gas or other diluent to control flame temperature andthus influence NO_(x) production.

In spite of their wide spread use, venturis still have certainlimitations. In the first place, the capacity of the venturi forinducing a flow of induced fluid is limited by the available pressure ofthe inducing fluid and the quantity of the latter needed for a givenapplication. In addition, the length of an efficient venturi typicallyis directly related to the diameter of the throat. The physicaldimensions of the work environment thus may have a limiting influence onthe capacity of the venturi.

In a more general sense, the reduction and/or abatement of NO_(x) inindustrial burners has always been a desirable aim. Some NO_(x)abatement has been achieved in the past by using a fuel lean primarycombustible fuel/air mixture coupled with staging of a portion of thegaseous fuel. Fuel lean primary mixtures are potentially desirable insome applications because the excess air provides a load to reduce flametemperatures whereby to reduce NO_(x). Staged gas may then be introducedinto the combustion zone either from gas tips arranged around theperiphery of the burner or from a center gas tip which protrudes throughthe center of the downstream end of the burner nozzle. The secondaryfuel is combusted with the excess air in an environment where flue gasesare available as a diluent. These arrangements have not always beensuccessful in reducing NO_(x) to desirable levels.

In some instances, a fuel lean primary mixture is introduced into thecombustion zone at a relatively high velocity due to the extra massprovided by the excess air. Such velocity may sometimes be so high thatthe flame speed is exceeded providing an unstable flame environment.

SUMMARY OF THE INVENTION

In accordance with the principles and concepts of the invention, thesame provides, in one important aspect, a compound venturi structurewhich includes a venturi cluster made up of a plurality of venturis.Thus, by definition, in accordance with this aspect of the invention,the compound venturi structure has at least two venturis. Desirably, thestructure may have at least three, often will have at least six, and insome instances, depending upon the exigencies of a particularapplication, may have even more than six venturis. An important purposeof the present invention is to provide practical solutions for problemsthat are extant in the burner field today, in particular those thatinvolve the production of excessive NO_(x) levels. Thus, the inventionprovides structure and methodology directed to addressing andalleviating the problems which have been mentioned above. Moreover, theinvention solves problems that relate to venturis generally. Because ofthe increased surface area provided by the multiplicity of venturis, agiven volume of the inducing fluid may educe a greater flow of theinduced material. Moreover, for a given flow of inducing fluid, thethroats of the venturis in a bundle have smaller throats and thereforemay be smaller in length.

Each of the venturis of the cluster may have an inlet, a throat and anoutlet, and each may be arranged and adapted for causing the flow of aninduced material by passage of an inducing fluid therethrough. Thisaction creates, in each venturi, a respective mixture of inducedmaterial and inducing fluid, which mixture may then be discharged fromthe outlets of the respective venturis. The structure also may desirablyinclude a collector having an inlet end which is connected to andarranged in fluid communication with the outlets of the venturis. Thus,the respective mixtures of inducing fluid and induced materialdischarged from the outlets may be collected and intermixed to present asingle mixed stream for discharge from an outlet end of the collector.The induced material most often may be a fluid material; however, inaccordance with the broader aspects and contemplations of the invention,the induced material may be a solid flowable material, such as, forexample, a powder or a flake material.

The venturis of the compound venturi structure of the invention maydesirably, but not necessarily, be in the form of elongated, essentiallystraight tubes. Preferably, but not necessarily, the tubes may bearranged in essential parallelism relative to one another. The venturismay also have essentially the same physical capacity; however, this alsois not a necessary or critical feature of the invention, and in fact,there are many applications where it may be desirable for at least oneof the venturis of a given cluster to have a different physical capacitythan another of the venturis of that same cluster.

In another important aspect of the invention, the compound venturistructure may be a component of a novel burner assembly. In accordancewith this aspect of the invention, in addition to the venturi clusterand the collector, the burner assembly may include a burner tip that isattached to and in fluid communication with an outlet end of thecollector. Thus, the tip may be arranged for receiving the single mixedstream of fluids from the collector and directing the same into acombustion zone.

In one important embodiment of the invention, the tip may be elongatedand adapted and arranged for directing the single mixed stream out ofthe tip and into the combustion zone in a generally radial directionrelative to a longitudinal axis of the tip. Such a tip may desirably beconfigured so as to create a round flat flame which surrounds the tip.

In another important embodiment of the invention, the tip may beelongated and adapted and arranged for directing the single mixed streamout of tip and into the combustion zone in a generally axial directionrelative to a longitudinal axis of the tip. This tip may desirably beconfigured so as to create a cylindrical flame which extends along theaxis.

In a general sense, either a gaseous fuel or air may be the inducingfluid; however, desirably, at least one of the venturis may be adaptedand arranged for operation with a gaseous fuel as the inducing fluid.When a gaseous fuel is used as the inducing fluid, either air orrecirculated flue gas may be the induced fluid. Desirably, at least oneof the venturis may be adapted and arranged to operate with air as theinduced fluid. Thus, when a gaseous fuel is used as the inducing fluidand air is the induced fluid, the single mixed stream created in thecollector may comprise a mixture of fluid fuel and air. Similarly, whena gaseous fuel is used as the inducing fluid and recirculated flue gasis the induced fluid, the single mixed stream may comprise a mixture offluid fuel and flue gas. For some applications, a gaseous fuel may beused as the inducing fluid to induce a flow of air in one venturi of agiven cluster and to induce a flow of flue gas in another venturi of thecluster. The single mixed stream may thus comprise a mixture of fluidfuel, air and recirculated flue gas. One or more of the venturis of thecluster may be adapted and arranged to operate with a diluent as theinduced fluid, whereby the single mixed stream comprises a fluid fueland a diluent. The diluent may be steam or nitrogen or CO₂ or some otheravailable gas which is inert relative to the combustion reactionprocess.

In accordance with an important aspect of the invention, the collectormay preferably be elongated and arranged so as to include a central axiswhich extends between the ends thereof. Desirably, the assembly may alsoinclude a central fuel tube that extends through the collector along theaxis of the latter. Ideally, the central fuel tube may also extendthrough the burner tip and the same may have a downstream end portionwhich projects through a centrally located opening at a downstream endof the burner tip. In accordance with a preferred aspect of theinvention, the assembly may include a fuel nozzle located at thedownstream end portion of the central fuel tube.

Ideally, the inlet end of the collector may include a respective opensegment for each of the venturis of the cluster, and the outlets of theventuris may each be connected to a respective segment. The segments maybe arranged in a series extending around the central fuel tube so thatthe mixed streams are evenly distributed around the interior of thecollector. If the tip is adapted and arranged for directing the singlemixed stream out of the tip and into the combustion zone in a generallyradial direction relative to a longitudinal axis of the tip, the fuelnozzle may desirably be adapted and arranged for providing secondaryfuel to the combustion zone. On the other hand, if the tip is adaptedand arranged for directing the single mixed stream out of the tip andinto the combustion zone in a generally axial direction relative to alongitudinal axis of the tip, the fuel nozzle may desirably be adaptedand arranged to provide a continuous primary flame at a location in thezone which is spaced axially from the downstream end of the tip.Ideally, in the latter case, the fuel nozzle may be located at aposition where it is spaced far enough from the downstream end of thetip in the combustion zone such that the single mixed stream has beenallowed to expand and slow to a speed such that its velocity, when itcomes into proximity with the fuel nozzle, is no greater than the flamesustaining velocity.

In another aspect, the invention provides a burner assembly thatcomprises a burner tube structure which may, but does not necessarily,include one or more venturi tubes. The burner tube structure does,however, include an elongated burner conduit having spaced inlet andoutlet ends. Such conduit may be a venturi tube. Alternatively it maysimply be a hollow tube or pipe. The conduit may generally be adaptedand arranged for directing a combustible gaseous mixture comprising afluid fuel, preferably in the form of a gaseous fuel, and oxygen,preferably in the form of air, therealong from the inlet end thereof tothe outlet end. In accordance with this aspect of the invention, aburner tip may be provided at the outlet end of the conduit, and suchburner tip may desirably have a central axis and a downstream end spacedfrom the outlet end of the conduit. The tip may generally be arrangedand adapted for receiving the combustible mixture from the conduit anddirecting the same through one or more apertures at the downstream endof the tip and into a combustion zone in a direction generally along theaxis of the tip.

The assembly of this aspect of the invention may further include anelongated central fuel tube that extends through the tip and along theaxis. This fuel tube desirably may project out of the tip in an axialdirection through the downstream end of the latter, and the fuel tubemay have a downstream end portion that is located in the combustion zonein spaced relationship relative to the downstream end of the burner tip.The aperture or apertures at the downstream end of the tip maybedisposed around the fuel tube, whereby the mixture directed into thecombustion zone may generally be in the form of a cylinder whichsurrounds the fuel tube and extends outwardly of the downstream end ofthe tip along the axis toward the downstream end portion of the fueltube. Ideally, the assembly includes a fuel nozzle on the downstream endportion of the fuel tube which is located at a position in the zone thatis sufficiently remote from the downstream end of the burner tip so asto permit the mixture to expand after it has left the downstream end ofthe tip and slow to a velocity which is less than the flame velocitythereof before it comes into proximity with the fuel nozzle. In thisform of the invention, the burner assembly may desirably be used insituations where the combustible mixture comprises an ultra fuel leanmixture of fuel and air.

In further accordance with the concepts and principles of the invention,a generally dome shaped burner tip is provided. The novel burner tip ofthe invention desirably includes a generally ring shaped base portionhaving a central axis and a plurality of elongated, side-by-side,circumferentially spaced, longitudinally curved ribs which extend in adirection along the axis. The ribs may each have a first end that ismounted on the base and a second end that is spaced from the base, withthe second ends being located nearer the axis than the first ends. Thebase portion and the ribs together define an area inside the tip adaptedfor receiving a flow of a mixture of air and fluid fuel, and the ribsalone define a multiplicity of curved slots therebetween permitting themixture to flow from the area inside the tip and outwardly into acombustion zone outside the burner tip in both a radial direction and ina direction which includes a vector extending along the axis. Inaccordance with the invention, the burner tip may comprise a crownportion connected to the second ends of the ribs, and such crown portionmay include a plurality of axially and radially extendingdiscontinuities which are aligned with respective slots such that theair/fluid fuel mixture flowing through the discontinuities has a morepronounced axial flow direction relative to the air/fluid fuel mixtureflowing through the slots. These discontinuities may desirably bepositioned so as to cause the air/fluid fuel mixture flowingtherethrough to create a prestaged mixing area outside the combustionzone. The crown portion may also have an axially aligned, gas nozzleaccommodating opening therein.

In one preferred embodiment of the invention, the tip described in theforegoing paragraph may be used in conjunction with a burner assemblythat comprises a compound venturi structure as described above.

The invention also provides a method for increasing the capacity of aventuri device to induce the flow of a second fluid into a first fluidwhen a flow of the first fluid passes through the device. The methodcomprises separating the first fluid into at least two, desirably atleast three, perhaps at least six or more separate flow portions,passing each separate flow portion of the first fluid through arespective venturi to independently induce a flow of the second fluidinto each of the flow portions thereby creating respective separatemixtures of the first and second fluids, and admixing the respectiveseparate mixtures to thereby create an admixture of the first and secondfluids containing a greater concentration of the second fluid than wouldbe possible by passing the entire amount of the first fluid through asingle venturi. In accordance with the invention, the first fluid maydesirably be a gaseous fuel and the second fluid may desirably be air.

The invention further provides a method for decreasing the length of aventuri device adapted for inducing the flow of a second fluid into afirst fluid when a flow of the first fluid is passed through the device.In this form of the invention, the method comprises separating the firstfluid into at least two, preferably at least three, and perhaps at leastsix or more separate flow portions; passing each separate flow portionof the first fluid through a respective venturi to independently inducea flow of the second fluid into each of the flow portions of the firstfluid, thereby creating respective separate mixtures of the first andsecond fluids; and admixing the respective separate mixtures to therebycreate an admixture of the first and second fluids containing a greaterconcentration of the second fluid than would be possible by passing theentire amount of the first fluid through a single venturi of the samelength.

Furthermore, the invention provides a method for operating a venturidevice that comprises providing at least two venturis, each venturihaving an inlet, a throat and an outlet, and each being operable forinducing the flow of an induced material when an inducing fluid ispassed therethrough, whereby to produce a respective mixture of theinduced material and the inducing fluid and discharging the mixture fromthe outlet thereof; passing a first inducing fluid through a first ofthe venturis to thereby induce the flow of a first induced material andproduce a first mixture comprising the first inducing fluid and thefirst induced material, and discharging the first mixture from theoutlet of the first venturi; passing a second inducing fluid through asecond of the venturis to thereby induce the flow of a second inducedmaterial and produce a second mixture comprising the second inducingfluid and the second induced material, and discharging the secondmixture from the outlet of the second venturi; and collecting andintermixing the first and second mixtures to present a single mixedstream of the fluids and materials.

Additionally the invention provides a method for operating a burnerequipped with a venturi device for supplying a combustible mixture to aburner nozzle which comprises providing at least two venturis, eachventuri having an inlet, a throat and an outlet, and each being operablefor inducing the flow of an induced fluid when an inducing fluid ispassed therethrough, whereby to produce a respective mixture of theinduced and inducing fluids that is discharged from the outlet thereof;passing a first inducing fluid through a first of the venturis tothereby induce the flow of a first induced fluid and produce a firstmixture comprising the first inducing fluid and the first induced fluid,and discharging the first mixture from the outlet of the first venturi;passing a second inducing fluid through a second of the venturis tothereby induce the flow of a second induced fluid and produce a secondmixture comprising the second inducing fluid and the second inducedfluid, and discharging the second mixture from the outlet of the secondventuri; and collecting and intermixing the first and second mixtures topresent a single combustible mixed stream of the fluids. Ideally, thefirst and second inducing fluids may each be gaseous fuels and the firstand second induced fluids may each be air. Alternatively, the firstinduced fluid may be air and the second induced fluid may be arecirculated flue gas or other diluent such as steam or nitrogen or CO₂or any other inert gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of a burner assembly which includes acompound multiventuri cluster that embodies the concepts and principlesof the invention;

FIG. 2 is a view, similar to FIG. 1, except that the assembly is shownpartly in cross-section to reveal the interior components;

FIG. 3 is a top plan view of the burner assembly of FIG. 1;

FIG. 4 is a cross-sectional view taken essentially along the line 4-4 ofFIG. 2;

FIG. 5 is a cross-sectional view taken essentially along the line 5-5 ofFIG. 2;

FIG. 6 is an elevational view, partly in cross-section, illustrating aportion of an alternative compound multiventuri cluster that embodiesthe concepts and principles of the invention;

FIG. 7 is an enlarged detail view illustrating the encircled portion 7of the compound venturi cluster of FIG. 6;

FIG. 8 is a perspective view of an embodiment of a burner tip whichembodies the concepts and principles of the invention and which maybeused in conjunction with a compound venturi cluster of the invention topresent a burner assembly;

FIG. 9 is a top plan view of the burner tip of FIG. 8;

FIG. 10 is a perspective view of an alternative embodiment of a burnertip which embodies the concepts and principles of the invention andwhich may be used in conjunction with a compound venturi cluster of theinvention to present a burner assembly;

FIG. 11 is a schematic view of another embodiment of a burner assemblywhich embodies the concepts and principles of the invention;

FIG. 11A is a partial view showing an alternative arrangement for theburner assembly of FIG. 11;

FIG. 12 is a schematic view of yet another embodiment of a burnerassembly which embodies the concepts and principles of the invention;

FIG. 13 is an elevational view, partly in cross-section illustrating thedownstream portion of yet another burner assembly which embodies theconcepts and principles of the invention;

FIG. 14 is an enlarged cross-sectional view illustrating the details ofthe burner assembly portion of FIG. 13;

FIG. 15 is a top plan view of the burner assembly of FIG. 13; and

FIG. 16 is a schematic view of a burner assembly similar to the burnerassembly of FIGS. 11 and 11A except that the center venturi bundle issurrounded by the peripheral venturi bundle.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The present invention provides a number of novel features which areuseful either in combination or alone. In particular these features areuseful in connection with burners and/or burner assemblies adapted toburn fluid fuels. These fluid fuels may be fuel oil or the like, butpreferably may be a gaseous fuel such as natural gas, propane, butane orhydrogen, or the like.

One burner assembly which embodies principles and concepts of theinvention is illustrated in FIGS. 1 through 5, where it is identified bythe reference numeral 20. The burner assembly 20 includes an outer,generally cylindrical shell 22 and a series of peripherally mountedsecondary fuel nozzles 24 that are connected to a fuel manifold 26. Ascan be seen in FIGS. 2, 4 and 5, the assembly 20 further includes acompound venturi structure 28 which, as shown, includes a venturicluster 30 made up of six separate and discrete venturis 32. Each of theventuris 32 has an inlet 34 at the lower or upstream end thereof (as thesame are depicted in FIG. 2), a throat 36, and an outlet 38 at its upperor downstream end. As can be seen in FIGS. 2, 4, 6 and 7, the inlet endportions 35 of the venturis, which extend from the inlets 34 to thethroats 36, are outwardly flared and essentially cone or bell shaped.

Individually, the venturis 32 may be conventional venturi typestructures of the sort that are well known to the routineers in theburner art, and the same may each be adapted and arranged so as to causethe flow of an induced material simply by passing an inducing fluidtherethrough. By this phenomenon, a respective mixture of inducedmaterial and the inducing fluid is created in the venturi and dischargedthrough the outlet 38 at the downstream end of the venturi.

The structure 28 also includes a collector 40 having an inlet end 42that, as shown in FIG. 2, is connected to and arranged in fluidcommunication with the outlets 38 of the venturis 32. As will beappreciated by those skilled in the art, the upper ends 39 of theventuris 32 adjacent the outlets 38 thereof may have an appropriateshape, as shown schematically in FIG. 5, so as to provide a smoothtransition zone 41 where the outlets 38 join the inlet end 42 of thecollector 40. By virtue of such an arrangement, the respective mixturesleaving the outlets 38 at the downstream ends of the venturis arecollected and intermixed in the collector 40 to thereby form a singlemixed stream. To facilitate the intermixing operation, the collector 40may be provided with a radially expanded portion 43 as shown.

Although the venturi cluster is depicted in FIGS. 2, 4 and 5 asincluding six separate venturis, it will be apparent to those skilled inthe art that the cluster may just as well include as few as two venturisarranged for parallel flow. Conversely, the cluster may include evenmore than six venturis, for example twelve or more venturis, dependingupon the needs of a given application.

As would be apparent to those of ordinary skill in the art, the inducingfluids for the venturis may be different. Also, the induced materials donot need to be all the same. For example, in the case of a burner, theinducing fluid may be a fuel such as a gaseous fossil fuel or hydrogen,while the induced material may be a fluid such as, for example, air, ora combustion-inert diluent, such as, for example, recirculated flue gas,steam, CO₂ or nitrogen. Alternatively, the inducing fluid may be airwhile the induced material may be a fluid fuel or a diluent. In anycase, the respective mixtures produced in the individual venturis 32will become intimately intermixed in the collector 40 so as to produce asingle mixed stream which, when the venturi cluster 30 is used in aburner, may contain an oxidant, a fluid fuel, and an appropriatediluent.

For purposes of using the concepts and principles of the presentinvention in connection with burners, the inducing fluid may desirablybe a fluid, preferably a gaseous fuel, and the induced material maydesirably be an oxygen containing gas, preferably air. To this end, theburner assembly 20 may be provided with a series of fuel gas inlet tubes44 that may be connected to a common source of fuel which is not shownin the drawings. The burner assembly 20 may also be provided with aseries of control handles 46, desirably one handle 46 for each venturi32. These handles 46 are each operable for moving a respective controlelement 48 in a conventional manner, toward and away from the inlet 34of a corresponding venturi 32, to thereby control the amount of airwhich may be drawn into the corresponding venturi 32 from a surroundingair box as a result of pressurized fuel gas flowing into the inlet 34via inlet tube 44. The air box is indicated generally by the referencenumeral 50 in FIG. 1.

With the arrangement described above, when fuel gas is discharged into arespective venturi through a corresponding tube 44, air from air box 50is drawn into inlet 34 through the gap 52 between each inlet 34 and thecorresponding element 48. The amount of air drawn into inlet 34 may becontrolled by varying the width of the gap 52 by raising and/or loweringthe element 48 using the corresponding handle 46. This air, which isdrawn into inlet 34 as a result of the fuel gas flowing into inlet 34via tube 44, joins with the fuel gas discharged from tube 44 to therebycreate a mixture of fuel gas and air that flows through the venturi 32and is discharged from the venturi 32 via outlet 38.

The details of the air controls are particularly well illustrated inFIGS. 6 and 7, where the same are shown as components of a three venturiburner arrangement. It is to be noted in this latter regard, that theventuris 32, the tubes 44, the handles 46 and the control elements 48 ofFIGS. 6 and 7 are essentially the same as the corresponding componentsof the assembly 20 of FIGS. 1 and 2. Thus, as the handles 46 are turnedin on direction, the gap 52 is widened, and when turned in an oppositedirection, the gap 52 is narrowed. The arrangement of FIGS. 6 and 7 alsoincludes a central fuel supply pipe 70 which serves purposes discussedherein below.

The individual respective mixtures from the venturis 32 are collectedand intermixed in collector 40 to present a single mixed stream of fuelgas and air which may then be directed into a burner tip 54 fordistribution into a combustion zone 56 that generally surrounds theupper end 58 of the burner arrangement 20. As can be seen in FIG. 2, thecollector 40 may preferably be elongated in a direction along thecentral longitudinal axis 60 of the burner assembly 20, and the same mayhave a outlet or downstream end 62 upon which the burner tip 54 may bepositioned.

The venturis 32 may preferably be arranged for parallel flow in thecluster 30, and the respective mixtures produced in the venturis are fedinto a common collector 40 where the same are joined together to presenta single combustible premix comprising air and fuel. This premix is thendirected into the common premix tip 54 which is mounted at thedownstream end 62 of the collector. The premix tip 54 may be designed insuch a way that the pressure inside the tip is essentially the same asthe pressure which would normally be present if only a single venturiwere employed. This insures a pressure drop associated with the velocityof the gas which is consistent with that associated with a singleventuri. The use of the multiple venturis allows for the use of multiplegas spuds (injectors) which in turn diffuse air into the simple gas jetat the same rate. The added surface area of three singular jets (or moredepending on the particular needs of a given application) allows forappreciable increases of air to be diffused into the jet. This alsoallows more air to be entrained into the opening of the venturi by themomentum of the jets because the entrainment rate of the induced fluidvaries directly with the surface area of the inducing stream. Theadditional air entrained is a function of the number of gas jetsemployed as well as the momentum of the gas once it leaves the spud(injector).

In one of the embodiments of the invention, as described above inrelationship to FIGS. 1, 2 and 3, the venturi cluster 30 may include sixof the venturis 32. In other, equally valuable forms of the invention,the cluster 30 may include two or more, three or more, or even more thansix venturis. For example, a burner assembly employing three venturis isillustrated in FIGS. 6 and 7. The only limitation in this regard is thatthe venturis of each cluster discharge into a common collector 40 wherethe individual mixtures from the respective venturis may be mixedtogether to form a single mixed stream. In the embodiment describedabove, the inducing fluid is described as being a fuel gas and theinduced fluid is described as being air.

In a preferred form of the invention, the respective capacities of theindividual venturis may be the same. In accordance with the broadcontemplation of the invention, however, the individual venturis of agiven cluster need not be identical. That is to say, the capacity of oneor more of the venturis of a given cluster may be different than thecapacity of one or more other venturis of the same cluster. Moreover,the inducing fluid of one or more of the venturis of a given cluster maybe different than the inducing fluid of one or more other venturis ofthe same cluster. In addition, the induced fluid of one or more of theventuris of a given cluster may be different than the induced fluid ofone or more other venturis of the same cluster. Just as an example inthis regard, the induced fluid of one venturi of a given cluster may beair, while the induced fluid of another venturi of the same cluster maybe flue gas or a diluent such as nitrogen or steam. Furthermore, and asanother example for a burner, the inducing fluid could be air and theinduced fluid could be a fuel gas. As would be readily appreciated bythose of ordinary skill in the burner art, there are a great number ofpossible combinations of venturi capacities, inducing fluid and inducedfluid which might be usefully employed in a single venturi cluster inaccordance with the concepts and principles of the invention.

The number of venturis to be used at any given time for any givenapplication is determined by the heat release of the burner as well asthe geometry of the burner which is desired for the application. Inultra low NO_(x) applications, one or more venturis may be utilized topull flue gases from the furnace, while the remaining venturis may beutilized for gas and air. The furnace flue gases may then be commingledwith the fuel and air mixture from the other venturis in the collector40, thus adding mass to the overall combustion stream. The additionalloading of the flame caused by the additional mass, along with thedeceleration of reaction kinetics, will lower the flame temperature thuslowering the NO_(x) emissions. This concept, along with the use of ahomogenous premixed mixture of gas and air as the primary fuel elementin other burner designs, can well lead to the reduction in NO_(x)emissions in other types of burners as well provide a broad range ofheat releases.

The use of a multiplicity of venturis to supply a premix of fuel and airfacilitates the provision of an ultra fuel lean premix. Such ultra fuellean premix may desirably contain only about 55% or so of the total fuelrequired, and perhaps even less, while often containing all of theoxygen required to combust the total fuel. The remainder of the fuel maythen be supplied as secondary fuel via staged nozzles. This concept ofultra lean premix, which keeps the gas to air ratio just above the lowercombustion limits, provides for maximum loading on the heat generated bythe primary flame. The multiple venturi arrangement facilitates theultra lean premix concept while maximizing the capability of staging arich raw gas stream as staged gas. The diffuse premix gas stream coupledwith flue gas entrained by the staged gas jets has opened up newopportunities for NO_(x) reduction. NO_(x) emissions performance in thisdesign of burner, has been observed to be as low as 3 ppm by vol.

As stated before, the surface area of multiple jets that are separatedand contained in independent converging bell shaped inlets 35, isillustrated in FIGS. 2, 4, 5 and 6, is much more efficient in entrainingair. This is due to the additional jet surface area and decrease indiameter created by separating one large jet into several small jets.The size of the jet is decreased as a function of the diameter of theport and the divergence of the jet in the ambient fluid. The angle ofdivergence, which is largely a function of the design of the gas port,also is determinative of the surface area of the jet. Each jet createdusing separate gas ports and multiple venturis, when supplied at thesame fuel pressure, will entrain and diffuse air at the same rate. Thisrate of diffusion/entrainment will increase the burner=s capability ofdelivering a very fuel lean or ultra lean premix. Although not desired,the composition of the premix from the multiventuri cluster can beadjusted to the point where the mixture is below flammability limits. Bykeeping the premix composition just within the flammability limits ofthe fuel being fired, it is possible maximize the mass of air that willthen maximize the thermal load on the flame. The additional thermal loadwill decrease the flame temperature and thus reduce thermal NO_(x)formation.

Another embodiment of a burner assembly which embodies the principlesand concepts of the invention is illustrated schematically in FIG. 11where it is identified by the reference numeral 120. In FIG. 11,components which are essentially the same as components identified inconnection with FIGS. 1 through 5 are given like reference numerals. InFIG. 11, the venturi cluster 30 is shown as having only two venturis 32;however, as explained above, the cluster 30 of FIG. 11 could just aswell have three or four or more venturis, with the only limitation beingavailable space. The venturis 32, as shown in FIG. 11, each include anelongated, essentially straight tube 64 which extends between the throat36 and the outlet 38. And as can be seen, the tubes 64 are arranged inessential parallelism relative to one another. In particular the tubes64 are arranged for parallel flow of fluids. It should be noted in thislatter regard, however, that the arrangement shown in FIG. 11 is notessential for the performance of the cluster 30. Rather, as will berecognized by those skilled in the art, it is not a necessity that thedownstream portions 64 of the venturi be straight or that the same bepositioned in parallelism relative to one another.

The burner tip 154 of the burner assembly of FIG. 11, which isillustrated in greater detail in FIG. 10, is preferably elongated in adirection along the axis 60 and the same is adapted and arranged fordirecting the single mixed stream of fuel and air received fromcollector 40 outwardly into zone 56 in a direction along axis 60. Tothis end, the tip 154 may be provided with a plurality of openings 66 inthe downstream end 67 thereof, which openings 66 are positioned todirect the mixed stream of fuel and air along the axis 60 as can best beseen in FIG. 11.

The venturis 32, as shown in FIG. 11, are each provided with a supply offuel gas via an inlet pipe or spud 68, and the air flow may becontrolled in the same manner as described above using moveable controlelements 48 (handles 46 not shown in FIG. 11). Thus, in the FIG. 11embodiment, fuel gas may be the inducing fluid and air is the inducedfluid. The assembly 120 of FIG. 11, may also be provided with anelongated central primary fuel tube 70 which extends along the centralaxis 60 of assembly 120 as shown and protrudes through a hole 69 in thedownstream end 67 of tip 154. A small venturi 72 is provided at theupstream end 74 of tube 70, and a supply of primary fuel for tube 70 isprovided via an inlet fuel spud or pipe 76. Thus, a primary mixture ofair and fuel is caused to flow along tube 70 toward a primary nozzle 78located atop a downstream end portion 80 of tube 70 that is located incombustion zone 56. It should be noted here, that in accordance with theinvention, while the material supplied to the nozzle 78 may desirably bean air/fuel premix, it is also possible that raw fuel may be supplied tothe nozzle 78 for stabilization purposes.

As can be seen from FIG. 11, the openings 66 are disposed in surroundingrelationship relative to tube 70. Thus, as the combustible mixture offuel and air is expelled from the tip 154 through the openings 66, thesame is in the form of a cylinder which extends toward nozzle 78 insurrounding relationship to tube 70. Upon ignition of the combustiblemixture, a generally cylindrical flame, which extends along axis 60, iscreated.

A flame holder 82, for a purpose discussed hereinafter, is mounted ontube 70 just beneath nozzle 78. The details of certain preferredembodiments of the flame holder 82 and the nozzle 78 are shown in FIGS.13 and 14. However, it is to be noted that the burner tip 254illustrated in FIG. 13 differs from the burner tip 154 of FIG. 11, inthat the latter has a plurality of the openings 66 in the end wall 156thereof, whereas the burner tip 254 simply has a cylindrical shape whichis essentially wide open at its downstream end 256.

With reference to FIG. 13, the flame holder 82 may desirably have aconical shape with the apex 84 thereof pointed away from the nozzle 78.Desirably, the apex 84 may be located approximately 8 inches above theupper end 256 of tip 254. In a particularly preferred form of theinvention, the flame holder 82 may have an outer diameter of about4≅when the tube 70 is formed from a 1≅diameter pipe, and the same maybeformed from a shaped plate attached to the tube 70 by tack welds or setscrews or the like. The enclosed angle α between the axis 60 and theskirt 83 of the cone of the flame holder 82 may desirably be 45E.

In its most preferred form, the holder 82 may have a plurality of3≅holes 86 therein distributed in a pattern which surrounds tube 70.These holes 86 may ideally be of sufficient size and number such thatapproximately 30 percent of the surface area of the holder 82 is openarea. It should be noted in this regard, however, that in accordancewith the principles and concepts of the invention, the open area mayrange from less than about 10% to more than about 75% of the surfacearea of the holder 82. For that matter, in accordance with theinvention, the holder may be of a variety of different diametersdepending upon the diameter of the main burner opening into the furnace.The diameter of the holder 82 may thus vary from one-fourth of thediameter of the main burner opening into the furnace to the samediameter as the main burner opening into the furnace. Further, the angleα may range from about 30E or less to about 80E or more. It should alsobe noted in connection with the foregoing, that the shape of the holder82 is not critical, and almost any shape may be used so long as the sameis capable of deflecting the combustible mixture leaving the tip 154,254 and create a low pressure 300 downstream of the flame holder 82which serves to pull the combustible mixture into a stagnant, lowvelocity zone where ignition may be stabilized and maintained.

The nozzle 78 may desirably be in the form illustrated in FIG. 14 whereit is shown as comprising a base 88 made up of a drilled and machinedpiece of hexagonal bar stock and a cylindrically shaped upper cupportion 90 having an open upper end 92. The base 88 may be provided withholes 94 and the cup portion 90 may be provided with holes 96, whichholes 94, 96 may be sized and positioned as necessary to achieve theresults desired for the nozzle 78 in providing the desired primaryflame. The cup portion 90 protects the flame from being blown off of theopen end 87 of the base 88 by ambient gas currents. In the absence ofsuch currents, the cup portion 90 may not be needed.

The arrangement illustrated schematically in FIGS. 11, 13 and 14provides extremely good NO_(x) performance. As explained above, themulti-venturi concept enables the provision of an ultra fuel lean premixwhich itself leads to substantial NO_(x) reduction. When themulti-venturi concept is coupled with the arrangement of FIGS. 11, 13and 14, even lower NO_(x) may be achieved as a result of the lowvelocity zone stabilization of the ultra-lean premix.

With reference again to FIG. 11, it is often preferred that only a smallportion, perhaps no more than about 10%, and desirably 2% or less, ofthe total fuel is introduced via spud 76 and used to educe air from airbox 50. The fuel and air are premixed in tube 70 coming up through thecenter of the burner. As mentioned above, in some cases it may bedesirable to supply a raw fuel via tube 70. Tube 70 passes through theprimary premix gas tip 154 and terminates in the shielded nozzle 78located some distance above the primary or main premix tip 154. Thisdistance may vary from less than about 3 inches to 15 inches or more,depending upon the speed and pressure of the premix as it leaves tip 154and the size of the burner. Thus a small primary flame is established inthe elevated nozzle 78 at a position above the upper end 156 of the mainpremix tip 154. The cone shaped flame holder 82, fabricated fromperforated plate, is located just under the elevated nozzle 78 toprovide a location for the main premix mixture from tip 154 to be drawninto the primary stabilizing flame created adjacent nozzle 78. Thus thecone 82 and the primary nozzle 78 provide a mechanism for maintaining astable flame in the ultra fuel lean premix supplied from tip 154. Once astable flame has been established, the primary flame generated at theexit end 92 of nozzle 78 may be extinguished to provide even greaterNO_(x) reduction.

Locating the primary flame in the manner described above at asubstantial distance from the exit of the main burner tip 154 providesan opportunity for the main air/fuel mixture to expand and slow downafter exiting the main tip 154. This slowing down of the premix to aspeed no greater than the flame speed is desirable for stabilizing theultra fuel lean premix flame. A significant problem occurring when anultra fuel lean combustible mixture is used, is that flame speed variesdirectly with the fuel content. Thus, the flame speed is very low in anultra fuel lean mixture. Mixture temperature may also affect flame speedwith higher temperatures resulting in higher flame speeds and viceversa. That is to say, when the combustible mixture is ultra fuel lean,whereby it contains a very large excess of air, the velocity of the flowcoming out of the main burner tip may exceed the flame speed, acondition which results in blowing of the flame off of the burner tip.

By delaying ignition until after the main fuel air mixture has exitedfrom the tip and has expanded into the furnace space, has slowed down invelocity, and has been incrementally heated by radiation from the hotsurroundings, a situation is created where the flame speed once againexceeds the flow velocity and the flame is therefore easily maintainedin a stable condition in the stabilizing zone provided by the elevatednozzle 78 and holder 82. The ignition and combustion of the main gas ina low velocity zone stabilization manner, at a substantial distance fromthe main premix tip outlet, produces previously unobtainable NO_(x)reduction performance, approaching 5 ppm on natural gas and even lessthan 3 ppm on a refinery blend fuel gas (e.g., 25% hydrogen, 25%propane, 50% methane). In addition to the foregoing, the already dilutefuel lean premix entrains furnace flue products after it exits the maintip and while it is expanding and slowing and thereby becomes even morediluted before ignition. This also contributes to the greater NO_(x)reduction.

In accordance with the arrangement illustrated in FIG. 11, it may bepossible to run the center venturi 72 quite lean but within stableflammability limits, and drive the surrounding multi-venturi/commoncollector 140 to very very lean mixtures which may even be below theflammability limit and will have to depend on the furnace temperature tocomplete the oxidation of the fuel.

In accordance with another aspect of the invention, the fuel/air mixturein tube 70 may be supplied by an cluster arrangement which includes aplurality of venturis 32. This arrangement is illustrated schematicallyin FIG. 11A. In this case, the overall assembly desirably includes twoseparate venturi clusters, an outer one which supplies an air/fuelpremix to the burner tip 154 and an inner one supplying an air/fuelpremix to tube 70. Another alternative arrangement where the innermultiventuri bundle is completely surrounded by the outer venturi bundleis illustrated schematically in FIG. 16. As shown in FIG. 16, the outerventuri bundle includes the venturis 32 and the common collector 140,while the inner venturi bundle includes the venturis 72 and the commoncollector 340. In these cases where the arrangement includes an innermultiventuri bundle positioned within an outer multiventuri bundle, theinner cluster may be operated within stable flammability limits and theouter cluster may be operated so as to provide an extremely fuel leanair/fuel premix so as to maximize the conditions needed for NO_(x)reduction. It is contemplated that this sort of an arrangement willfacilitate the construction of very large burners having as many as sixor more venturis in the inner bundle and as many as twelve or moreventuris in the outer bundle.

With reference now to FIG. 12, it can be seen that the principles andconcepts of the invention apply also to radiant burners where the premixis directed radially from the tip 354. In this regard, reference is madeto co-pending, co-assigned U.S. application Ser. No. 09/803,808, filedMar. 12, 2001, the entirety of the disclosure of which is herebyincorporated herein by specific reference thereto. Thus, the burnerassembly 320 shown schematically in FIG. 12 includes the burner tip 354,which is elongated in a direction which extends axially through theburner assembly, and the same is adapted and arranged for directing thesingle mixed stream received from the collector 40 into the combustionzone 56 in a generally radial direction relative to the axis 60. Thus,the burner tip 354 is adapted and arranged to create a round flat flamewhich surrounds the tip 354. With further reference to FIG. 12, theassembly 320 may also include a central tube 170 to supply secondaryfuel to the combustion zone via a nozzle 178

In a particularly preferred form of the invention, the burner tip 354may be in the configuration illustrated in FIGS. 8 and 9, where it canbe seen that the tip 354 has a generally ring shaped base portion 98 anda central axis 100. Furthermore, the tip 354 has a plurality ofelongated, side-by-side, circumferentially spaced, longitudinally curvedribs 102. The ribs 102 have respective first ends 104 that are mountedon the base portion 98, and respective second ends 106 that are spacedfrom the base portion 98. As can be seen, the second ends 106 arelocated nearer the axis 60 than the first ends 104. The ribs 102 and thebase portion 98 define an area 108 inside of the tip 354 that is adaptedfor receiving a flow of the single mixture of fuel and air from thecollector 40. The ribs 102 define a multiplicity of curved slots 110therebetween. As can be seen from FIGS. 8 and 9, these slots 110 arearranged and positioned such that the mixture in area 108 is permittedto flow from the area 108 and outwardly into the combustion zone 56outside the tip 354 in both a radial direction and in a direction whichincludes a vector extending along axis 60.

In the preferred form thereof illustrated in FIGS. 8 and 9, the tip 354may also include a crown portion 112 that is connected to the respectivesecond ends 106 of the tip 354. Desirably, the crown portion 112 mayinclude a plurality of axially and radially extending discontinuities114 which are aligned with certain of the slots 110 such that themixture leaving area 108 through the discontinuities 114 has a morepronounced axial flow direction than the mixture leaving area 108through the slots 110 themselves. Ideally, the discontinuities 114 maybe positioned to cause the axially directed mixture flowing therethroughto create a prestaged mixing area 116 (see FIG. 12) that is outside thecombustion zone 56 where the mixture of fuel and air flowing through thediscontinuities 114 may circle around in area 116 in a directionindicated by the arrows 115 so as to become diluted with flue gas beforereturning to the combustion zone to be combusted. In comparison, thedirection of flow of the premix flowing from slots 110 is schematicallyillustrated by the arrows 117. In a particularly preferred form of theinvention, the crown portion 112 of the tip 354 may be provided with anaxially aligned, central gas nozzle accommodating opening 118.

In one aspect, the invention provides a radiant wall burner whichincludes a compound venturi cluster and is therefor capable of achievinghigh heat releases with 100% premix. This has not been possible prior tothe present invention. In the past, the highest heat releases attainablewere around 1.7 MMBTU/h with secondary air. However, it is to be notedthat secondary air typically causes higher NO_(x) than when all of theair is supplied as an air/fuel premix in the venturi section. Thisbarrier has now been broken with the new design disclosed herein whichincludes a compound venturi cluster consisting of a plurality ofventuris arranged in a single cluster for parallel fluid flow.

The invention provides low NO_(x) with staged fuel, low noise in someconfigurations, staged gas jets entraining the flue gas external to theburner, prompt NO_(x) alleviation, simplicity of operation with nosecondary air adjustments, short flame profile, high turndown ratioswith added premix tip velocities, high stability, minimal CO emissions,cooler premix tip (with added mass flow and greater heat transfer), andminimal flashback problems with added tip velocity.

The invention relates to a multiventuri design which, among otherthings, may provide excess air for ultra fuel lean mixtures for premixapplications. In particular the invention may be useful either inconnection with radiant wall burners or with burners which provide anaxial flame. The invention is also useful in connection with largeprocess heater burners with the primary combustible mixture made up of100% or partial premix as a NO_(x) reducing mechanism. But it is also tobe noted that the multiventuri design of the invention has generalapplicability and can be extrapolated for general use whenever venturisare needed. In particular the multiventuri design of the inventionoperates to entrain more air than previously thought possible throughincreased mass transfer and diffusion. Moreover, the multiventuri designof the invention has beneficial application in typical tank and vesselventing, air handling, solids transportation and handling and anywherewhere a short venturi may be needed to move large masses of materials.

In the past, radiant wall burners were not capable of reaching heatreleases in excess of 1.5 MMBtu/h without the use of some other airsource. With the use of multiple venturis in parallel, heat releaseswell in excess of 10 MMBtu/h are attainable with the correct geometryand attention to detail making sure interaction between venturis isminimized.

In one configuration, in accordance with the invention, it is possibleto apply the invention to modular burners where venturi eductors may beadded to increase capacity or reduce NO_(x). In this concept, a burnermay be installed with multiple venturis and may be upgraded at a laterdate with additional venturis to increase capacity or add steam or fluegas or other inert gases to reduce NO_(x). In another configuration theinvention is not limited to using just flue gas as a diluent to reduceNO_(x) but can be used with any other diluent that adds mass to quenchthe flame. Such diluents may range from any inert gas such as nitrogenor steam or CO₂ to low BTU fuels like refinery PSA gas or other fuelladen vapor or gas streams with any percentage of combustible gastherein.

In other configurations, the present invention can be applied to manydifferent designs of process heater burners that may be mounted on thefloor or roof of the furnace instead of the sidewall. These may createflames that are free standing and round or flat or otherwise. They mayfunction in furnaces that do not require the wall to be heated by theflame.

In yet other configurations, instead of fuel being used as the motivefluid in one or more of the eductors, steam or other compressed diluentgases as characterized above maybe used as the motive fluid.

Typical radiant wall burners use the motive force of a single gas spudto entrain air from the atmosphere. This new concept of utilizingmultiple venturis or eductors in parallel adds a new dimension to thecombustion industry. The strong points of the present invention, whenapplied to burner technology, are as follows:

-   -   (1) Shorter flame due to better homogeneity of gas and air;    -   (2) Large turndown ratios are possible (10:1 as opposed to 3:1        for prior art devices);    -   (3) Lower noise around the burner;    -   (4) Tiles are not subjected to hot spots created by burning jets        piercing to the tile;    -   (5) With 100% premix no secondary register is required;    -   (6) Burner operation is very stable;    -   (7) Burner is capable of running substoichiometric without        flashback;    -   (8) Ability to lower both prompt and thermal NO_(x) with flue        gas injection and mixing;    -   (9) Staging of fuel is easily accomplished with single internal        or multiple radial tips;    -   (10) Flashback with volatile fuels is minimized with higher tip        velocities; and    -   (11) Much larger heat releases are achieved than previously        thought possible.

In accordance with the concepts and principles of the invention, aburner which includes the novel compound venturi cluster that is thesubject of the foregoing disclosure may be designed for firing upwardly,downwardly or horizontally. Moreover, the multiventuri burner of theinvention may be used for burning combustible liquids such as fuel oil.Accordingly, with minimal difficulty and with minimal physical changes,the burner may be applied to combination firing arrangements. It shouldalso be noted that the burner of the invention is readily adaptable to avariety of shapes. For example, the burner could be configured as arectangular or other desired shape, in place of the round flame designdescribed above.

It is also clear from the foregoing description that the inventioncontemplates the use of a venturi cluster in combination with a centralfuel tube providing either a fuel/air premix to a central primary flamenozzle or a pure fuel to a central nozzle supplying secondary fuel to acombustion zone.

It is also clear that the invention principles and concepts of theinvention may be applied so as to provide a large burner arrangementwhich may include an inner multi-venturi cluster located within an outermulti-venturi cluster.

The present invention provide s a number of novel features which areuseful either in combination or alone in connection with burners and/orburner assemblies adapted to burn fluid fuels. These fluid fuels may befuel oil or the like, but preferably may be a gaseous fuel such asnatural gas, propane, butane or hydrogen, or the like.

1. A method for increasing the capacity of a venturi device to inducethe flow of an induced material into an inducing fluid when a flow ofthe inducing fluid is passed through the device, said method comprising:separating said first fluid into at least two separate flow portions;passing each separate flow portion of said first fluid through arespective venturi to independently induce a flow of said inducedmaterial into each of said flow portions thereby creating respectiveseparate mixtures of said induced material and said inducing fluid; andadmixing the respective separate mixtures to thereby create an admixtureof said inducing fluid and said induced material containing a greaterconcentration of said induced material than would be possible by passingthe entire amount of said inducing fluid through a single venturi.
 2. Amethod as set forth in claim 1, wherein said inducing fluid is separatedinto at least three of said separate flow portions.
 3. A method as setforth in claim 1, wherein said inducing fluid is separated into at leastsix of said separate flow portions.
 4. A method as set forth in claim 1,wherein said inducing fluid is a gaseous fuel.
 5. A method as set forthin claim 4, wherein said induced material is air.
 6. A method fordecreasing the length of a venturi device adapted for inducing the flowof an induced material into an inducing fluid when a flow of theinducing fluid is passed through the device, said method comprising:separating said inducing fluid into at least two separate flow portions;passing each separate flow portion of said inducing fluid through arespective venturi to independently induce a flow of said inducedmaterial into each of said flow portions thereby creating respectiveseparate mixtures of said induced material and said inducing fluid; andadmixing the respective separate mixtures to thereby create an admixtureof said induced material and said inducing fluid containing a greaterconcentration of said induced material than would be possible by passingthe entire amount of said inducing fluid through a single venturi of thesame length.
 7. A method for operating a venturi device comprising:providing at least two venturis, each venturi having an inlet, a throatand an outlet, and each being operable for inducing the flow of aninduced material when an inducing fluid is passed therethrough, wherebyto produce a respective mixture of the induced material and the inducingfluid and discharging the mixture from the outlet thereof; passing afirst inducing fluid through a first of said venturis to thereby inducethe flow of a first induced material and produce a first mixturecomprising the first inducing fluid and the first induced material, anddischarging said first mixture from the outlet of said first venturi;passing a second inducing fluid through a second of said venturis tothereby induce the flow of a second induced material and produce asecond mixture comprising the second inducing fluid and the secondinduced material, and discharging said second mixture from the outlet ofsaid second venturi; and collecting and intermixing said first andsecond mixtures to present a single mixed stream of said fluids andmaterials.
 8. A method for operating a burner equipped with a venturidevice for supplying a combustible mixture to a burner nozzle, saidmethod comprising: providing at least two venturis, each venturi havingan inlet, a throat and an outlet, and each being operable for inducingthe flow of an induced fluid when an inducing fluid is passedtherethrough, whereby to produce a respective mixture of the induced andinducing fluids and discharging the mixture from the outlet thereof;passing a first inducing fluid through a first of said venturis tothereby induce the flow of a first induced fluid and produce a firstmixture comprising the first inducing fluid and the first induced fluid,and discharging said first mixture from the outlet of said firstventuri; passing a second inducing fluid through a second of saidventuris to thereby induce the flow of a second induced fluid andproduce a second mixture comprising the second inducing fluid and thesecond induced fluid, and discharging said second mixture from theoutlet of said second venturi; and collecting and intermixing said firstand second mixtures to present a single combustible mixed stream of saidfluids.
 9. A method as set forth in claim 8, wherein said first andsecond inducing fluids are each gaseous fuels.
 10. A method as set forthin claim 9, wherein said first and second induced fluids are each air.11. A method as set forth in claim 9, wherein said first induced fluidis air and said second induced fluid is a recirculated flue gas.
 12. Amethod as set forth in claim 8, wherein one of said induced fluids is adiluent.
 13. A method as set forth in claim 12, wherein said diluent issteam.
 14. A method as set forth in claim 12, wherein said diluent isnitrogen.
 15. A method for operating a burner comprising: delivering aflow of a combustible mixture comprising a fuel and air from a nozzle toa combustion zone at composition where the flame speed of the mixture islower than the velocity of the mixture as the latter exits the nozzle;allowing the mixture to expand and thereby slow to a velocity which isno greater than said flame speed; and igniting said mixture only aftersaid velocity which is no greater than said flame speed has beenachieved.
 16. A method as set forth in claim 15, wherein said mixture isultra fuel lean.
 17. A method as set forth in claim 10, wherein saidsingle combustible mixed stream of said fluids is fuel lean and the sameis delivered to a combustion zone at a velocity which exceeds the flamespeed of the mixture, said method further comprising: allowing the mixedstream to expand and thereby slow to a velocity which is no greater thansaid flame speed; and igniting said mixed stream only after saidvelocity which is no greater than said flame speed has been achieved.18. A method as set forth in claim 1, wherein said material is a fluid.19. A method as set forth in claim 1, wherein said material is aflowable solid.
 20. A method as set forth in claim 6, wherein saidmaterial is a fluid.
 21. A method as set forth in claim 6, wherein saidmaterial is a flowable solid.
 22. A method as set forth in claim 7,wherein said material is a fluid.
 23. A method as set forth in claim 7,wherein said material is a flowable solid.